Controlling Structural and Energetic Disorder in High-Mobility Polymer Semiconductors via Doping with Nitroaromatics

Abstract

Molecular doping has emerged as a powerful strategy to tune the charge transport properties of organic field-effect transistors (OFETs). However, the limited tool-box of molecular dopants and unresolved challenges of stability, uniformity of the doping, and matching the energy levels constrain the achieved OFET device performance and thwart the practical applications. Here, nitrofluorene (NF) acceptors are introduced as effective p-dopants in polymer OFETs, resulting in outstanding device performance of a standard commercial diketopyrrolopyrrole-thienothiophene (DPP-DTT) polymer. An ∼5-fold enhancement in the saturation field-effect mobility (up to ∼8 cm2 V–1 s–1) is realized in ambient air operations after doping with 2,4,5,7-tetranitrofluorenone. Importantly, the achieved effective mobility (which accounts for device nonideality) exceeds 6 cm2 V–1 s–1, which is among the highest values reported for polymer OFETs. The spectroscopic, microscopic, X-ray diffraction, and electrical investigations elucidate the role of NF dopants in mitigating charge carrier traps, lowering the contact resistance, and maximizing the structural order of the polymer films. Energetic disorder reduces significantly upon doping as revealed via variable temperature mobility measurements.

Directional Carrier Polarity Tunability in Ambipolar Organic Transistors Based on Diketopyrrolopyrrole and Bithiophene Imide Dual-Acceptor Semiconducting Polymers

“…DPP-based dual-acceptor polymers have several advantages in this aspect: (i) The two acceptors increase the electron affinity to facilitate electron transport properties. Combined with commonly used electron-donating flanking groups of DPP, it is easier to achieve ambipolar properties in OFETs; (ii) the flanking group variation of DPP is a simple yet effective strategy to fine-tune the charge carrier polarity as reported in our previous publication; (iii) the doping/additive strategy makes positive influences on the OFET device performance with DPP-based semiconductors. , In terms of these favorable features and considering the lack of studies on the carrier polarity tunability of ambipolar transistors through the additive strategy, we developed two new DPP-based dual-acceptor polymers by selecting BTI as another acceptor unit. By varying the flanking groups of DPP and through the additive strategy, the charge carrier polarity tunabilities of both ambipolar polymers in OFETs are studied in detail.…”

“…Second, DPP-based polymers tend to have ambipolar transport properties because the flanking groups are generally electron-donating or weak electron-accepting units. ,, Ambipolar semiconductors are more suitable candidates for the study of charge carrier polarity tunability because they allow both holes and electrons flowing in the conducting channel of OFETs under negative and positive gate voltage, respectively. Third, the ease of synthesis and high device performance make DPP-based polymers the model materials in OFET device engineering, for example, chemical doping/additive. − Luo et al reported that the addition of ionic NMe 4 I to the DPP and thienothiophene polymer can remarkably improve the hole mobility to 26.2 cm 2 V –1 s –1 . A series of characterizations indicate that the improvement is not caused by chemical doping but ascribed to the backbone coplanarity improvement for more favorable molecular packing and stronger π–π interactions .…”

“…A series of characterizations indicate that the improvement is not caused by chemical doping but ascribed to the backbone coplanarity improvement for more favorable molecular packing and stronger π–π interactions . For the same polymer, Ghamari et al found that the tetranitrofluorenone (TeNF) is an effective p-type dopant . TeNF can interact with some in-gap states of the polymer to migrate the deep traps and improve the polymer film microstructure to remove some shallow traps .…”

“…For the same polymer, Ghamari et al found that the tetranitrofluorenone (TeNF) is an effective p-type dopant . TeNF can interact with some in-gap states of the polymer to migrate the deep traps and improve the polymer film microstructure to remove some shallow traps . Both effects synergistically contribute to the significant enhancement of hole mobilities .…”

“…TeNF can interact with some in-gap states of the polymer to migrate the deep traps and improve the polymer film microstructure to remove some shallow traps . Both effects synergistically contribute to the significant enhancement of hole mobilities . In contrast, n-type dopants are rarely reported in OFETs to enhance the electron transport property of DPP-based semiconductors.…”